Place the oil, celery seed, cumin seed, and red pepper flakes in a small pan and place over medium-low heat. Cook until fragrant, about 2 minutes. Remove from heat and cool for 10 minutes.
Combine the spiced oil with the apple cider vinegar, rice wine vinegar, sugar, Dijon, salt, and pepper in a glass jar. Lid up, shake well, and set aside for 1 hour.
Finely shred the cabbage with a knife or mandoline and pace it in the bowl of a salad spinner. Grate the carrots through the large holed of a box grater and add to the cabbage. Thinly slice the onion (can also do on mandoline) and add to the cabbage and carrots. Sprinkle the vegetables with the sugar and salt, toss well, and leave at room temperature for 10-15 minutes, until veggies start to soften.
Transfer the cabbage mix to a large bowl and toss with the dressing. Refrigerate for at least 2 hours before serving.